Generation of leucogranites via fractional crystallization: A case from the Late Triassic Luoza batholith in the Lhasa Terrane, southern Tibet
Generation of leucogranites via fractional crystallization: A case from the Late Triassic Luoza batholith in the Lhasa Terrane, southern Tibet
复制标题
DOI：
10.1016/j.gr.2018.08.008
复制
发表时间：
2019-02
影响因子：
6.1
通讯作者：
Li-Xue Zhang;Qing Wang;Di‐Cheng Zhu;Di‐Cheng Zhu;Shi‐Min Li;Zhidan Zhao;L. Zhang;Yuelong Chen-Yuelon
中科院分区：
文献类型：
--
作者：
Li-Xue Zhang;Qing Wang;Di‐Cheng Zhu;Di‐Cheng Zhu;Shi‐Min Li;Zhidan Zhao;L. Zhang;Yuelong Chen-Yuelon
浅花岗岩对于理解地壳内分异和区域构造演化具有重要意义，但这些岩石是如何形成的仍是一个有争议的问题。拉萨中部的罗扎岩基由正常钙碱性s型花岗闪长岩（第1组）和高分异s型亮花岗岩（第2组）组成，为研究这些问题提供了重要的机会。激光烧蚀电感耦合等离子体质谱锆石U-Pb测年表明，这些岩石的就位时间约为221-205 Ma。第1组样品（ca. 205 Ma） sio2含量低（64 ~ 67 wt%），分化指标值低（DI = 75 ~ 76），Ba、P、Sr和Ti含量较低（ca. 221 Ma和213 Ma），后者sio2含量高（74 ~ 75 wt%），分化指标值高（91 ~ 94），Ba、P、Sr和Ti含量较低。第1组和第2组样品的锆石εHf(t)值相似（−15.6 ~−1.4），均显著高于拉萨中部纯古基底熔体（−20.5 ~−10.6）。第1组样品可以解释为来自于拉萨中部古基底内的偏长砾岩部分熔融，并有地幔或幼壳衍生成分的贡献，而第2组样品可能是由母岩浆中不同矿物（如斜长石 ± 钾长石、黑云母、锆石、allanite/monazite、钛矿和磷灰石）的不同程度的分离结晶而形成的。以第1组花岗闪长岩样品为代表，位于地壳浅岩浆房中。结合沉积记录，第1组样品所显示的高温低压条件可与班公-怒江-特提斯洋海底向南俯冲的弧后伸展环境相联系。我们的研究表明，分馏结晶可能是浅色花岗岩发育的一种可行机制，并解释了花岗岩在壳内分化过程中的成分多样性。
Leucogranites are important in understanding intracrustal differentiation and regional tectonic evolution, but how these rocks form remains a matter of much debate. The Luoza batholith in the central Lhasa subterrane provides an important opportunity to address these issues as this batholith consists of normal calc-alkaline S-type granodiorites (Group 1) and highly fractionated S-type leucogranites (Group 2). Laser Ablation Inductively Coupled Plasma Mass Spectrometry zircon U-Pb dating indicates that these rocks were emplaced at ca. 221–205 Ma. The Group 1 samples (ca. 205 Ma) have low SiO2content (64–67 wt%) and low differentiation index values (DI = 75–76), and are slightly depleted in Ba, P, Sr, and Ti compared to the Group 2 samples (ca. 221 Ma and 213 Ma), which have high SiO2content (74–75 wt%), high differentiation index values (91–94), and are significantly depleted in Ba, P, Sr, and Ti. Samples from Group 1 and Group 2 exhibit similar zircon εHf(t) values (−15.6 to −1.4), both of which are significantly higher than melts from the pure ancient basement in the central Lhasa subterrane (−20.5 to −10.6). The Group 1 samples can be interpreted as having been derived from the partial melting of metagreywacke within the ancient basement in the central Lhasa subterrane with contributions from mantle- or juvenile- crust-derived components, whereas the Group 2 samples may have formed via varying degrees of fractional crystallization of different minerals (e.g., plagioclase ± K-feldspar, biotite, zircon, allanite/monazite, titanite, and apatite) from the parental magmas, represented by the granodioritic samples in Group 1, in shallow crustal magma chambers. In combination with sedimentary records, the high-temperature and low-pressure conditions indicated by the Group 1 samples can be geodynamically linked to a back-arc extensional setting in response to the southward subduction of the Bangong-Nujiang Tethys Ocean seafloor. Our work implies that fractional crystallization is likely a feasible mechanism for the development of leucogranites and for explaining the compositional diversity of the granite during intracrustal differentiation.